All items that are not produced in the Laboratory or at the Assembly Plant have a random drop-rate, which means that there is a low chance to obtain them, e.g. you won't get an Engine or Weapon Parts from every destroyed enemy vehicle. Some items drop, especially event-related ones, may be also limited, e.g. you cannot get more than 10 Contracts in PVP per day.
You can read about the basic items in Warzone in this article.